Albumin is the most abundant plasma protein, generally constituting slightly over one-half of the total protein in mammalian plasma. In the human body, albumin has the important role of regulating the water balance between blood and tissues, and of functioning as a transport molecule for various compounds, such as bilirubin, fatty acids, cortisol, thyroxine and drugs such as sulfonamides and barbiturates, that are only slightly soluble in water. An albumin deficiency can restrict the transport of slightly water soluble materials throughout the body and a deficiency is signaled in an individual by an abnormal accumulation of serous fluid, or edema. Therefore, it is clinically important to determine whether an individual has a deficiency of serum albumin.
Likewise, it is clinically important to determine if an individual is excreting an excess amount of protein. A normal functioning kidney forms urine in an essentially two step process. Blood flows through the glomerulus, or glomerular region of the kidney. The capillary walls of the glomerulus are highly permeable to water and low molecular weight components of the blood plasma. Albumin and other high molecular weight proteins cannot pass through these capillary walls and are essentially filtered out of the urine so that the protein is available for use by the body. The liquid containing the low molecular weight components passes into the tubules, or tubular region, of the kidney where reabsorption of some urine components, such as low molecular weight proteins; secretion of other urine components; and concentration of the urine occurs. As a result, through the combined processes of the glomerulus and tubules, the concentration of proteins in urine should be minimal to non-existent. Therefore, abnormally high amounts of albumin and/or low-molecular weight proteins in urine must be detected and related to a physiological dysfunction.
The relatively high concentration of albumin in the urine of an individual usually is indicative of a diseased condition. For example, the average normal concentration of protein in urine varies from about 2 mg/dL to about 8 mg/dL, with approximately one-third of the total urinary protein being serum albumin. However, in a majority of diseased states, urinary protein levels increase appreciably, such that albumin accounts for from about 60 percent to about 90 percent of the excreted protein. The presence of an abnormal increased amount of protein in the urine, known as proteinuria, is one of the most significant indicators of renal disease, and may be indicative of various other non-renal related diseases.
Therefore, in order to determine if an individual has an albumin deficiency and/or to determine if an individual excretes an excess amount of protein, and in order to monitor the course of medical treatment to determine the effectiveness of the treatment, simple, accurate and inexpensive protein detection assays have been developed. Furthermore, of the several different assay methods developed for the detection and/or measurement of protein in urine and serum, the methods based on dye binding techniques have proven especially useful because dye binding methods are readily automated and provide reproducible and accurate results.
In general, dye binding techniques utilize pH indicator dyes that are capable of interacting with a protein, such as albumin, and that are capable of changing color upon interaction with a protein absent any change in pH. When a pH indicator dye interacts with, or binds to, a protein, the apparent pKa (acid dissociation constant) of the indicator dye is altered and the dye undergoes a color transition, producing the so-called "protein-error" phenomenon. In methods utilizing the dye binding technique, an appropriate buffer maintains the pH indicator dye at a constant pH to prevent a color transition of the pH indicator dye due to a substantial shift in pH. Due to the "protein-error" phenomena, upon interaction with the protein, the pH indicator dye undergoes a color transition that is identical to the color change arising because of a change in the pH. Examples of pH indicator dyes used in the dry phase assay of proteins that are capable of interacting with or binding to proteins and exhibiting "protein-error" color transitions include tetrabromophenol blue and tetrachlorophenol-3,4,5,6-tetrabromosulfophthalein.
Although pH indicator dyes have been used extensively in protein assays, several problems and disadvantages still exist in protein assay methods utilizing indicator dyes. For example, methods based upon pH indicator dyes either cannot detect or cannot quantitatively differentiate between protein concentrations below approximately 15 mg/dL. In addition, although several simple semiquantitative tests and several complex quantitative tests are available for the determination of the total protein content in a test sample, the majority of these assay methods, with the notable exception of the simple colorimetric reagent test strip, require the precipitation of protein to make quantitative protein determinations.
The colorimetric reagent test strip utilizes the previously discussed ability of proteins to interact with certain acid-base indicators and to alter the color of the indicator without any change in the pH. For example, when the indicator tetrabromophenol blue is buffered to maintain a constant pH of approximately 3, the indicator imparts a yellow color to solutions that do not contain protein. However, for solutions containing protein, the presence of protein causes the buffered dye to impart either a green color or a blue color to solution, depending upon the concentration of protein in the solution.
Some colorimetric test strips used in protein assays have a single test area consisting of a small square pad of a carrier matrix impregnated with a buffered pH indicator dye, such as tetrabromophenol blue. Other colorimetric test strips are multideterminant reagent strips that include one test area for protein assay as described above, and further include several additional test areas on the same strip to permit the simultaneous assay of other urinary constituents. For both types of colorimetric test strips, the assay for protein in urine is performed simply by dipping the colorimetric test strip into a well mixed, uncentrifuged urine sample, then comparing the resulting color of the test area of the test strip to a standardized color chart provided on the colorimetric test strip bottle.
For test strips utilizing tetrabromophenol blue, buffered at pH 3, as the indicator dye, semiquantitative assays for protein can be performed and are reported as negative, trace, or one "plus" to four "plus". A negative reading, or yellow color, indicates that the urine contains no protein, as demonstrated by the lack of a color transition of the indicator dye. A trace reading may indicate from about 5 to about 20 mg/dL of protein in the urine. The one "plus" to four "plus" readings, signified by color transitions of green through increasingly dark shades of blue, are approximately equivalent to urine protein concentrations of 30, 100, 300, and over 2000 mg/dL, respectively, and serve as reliable indicators of increasingly severe proteinuria.
In accordance with the above-described method, an individual can readily determine, visually, that the protein content of a urine sample is in the range of 0 mg/dL to about 30 mg/dL. However, the color differentiation afforded by the presently available commercial test strips is insufficient to allow an accurate determination of protein content in urine between 0 mg/dL and about 15 mg/dL. The inability to detect and differentiate between low protein concentrations is important clinically because a healthy person usually has a urine protein level in the range of about 10 mg/dL to about 20 mg/dL. Therefore, it could be clinically important to know more precisely the urine protein content of an individual, rather than merely estimating the protein content at some value less than about 30 mg/dL.
Of course, the protein content of a urine sample can be determined more precisely by semiquantitative protein precipitation techniques or by quantitative 24 hour protein precipitation techniques. However, these tests are time consuming and relatively expensive. Furthermore, the precipitation tests must be run in a laboratory by trained personnel, and therefore are unavailable for the patient to perform at home to quickly determine urine protein content and to monitor the success or failure of a particular medical treatment.
Therefore, it would be extremely advantageous to have a simple, accurate and trustworthy method of assaying urine for protein content that allows visual differentiation of protein levels in the ranges of 0 mg/dL to about 10 mg/dL, about 10 mg/dL to about 20 mg/dL, and about 20 mg/dL to about 30 mg/dL, and upwards to between about 100 mg/dL to about 300 mg/dL. By providing such an accurate method of determining urine protein concentration in an easy to use form, such as a dip-and-read test strip, the urine assay can be performed by laboratory personnel to afford immediate test results, such that a diagnosis can be made without having to wait up to one day for assay results and medical treatment can be commenced immediately. In addition, the test strip method can be performed by the patient at home to more precisely monitor low levels of protein in urine and/or the success of the medical treatment the patient is undergoing.
As will be described more fully hereinafter, the method of the present invention allows the fast, accurate and trustworthy protein assay of urine by utilizing a test strip that includes a dual indicator reagent composition. The dual indicator reagent composition improves the visual color resolution, and therefore the sensitivity, of the assay, thereby allowing urine protein concentrations to be accurately determined at levels of approximately 30 mg/dL or less. In addition, the method of the present invention can be used to determine the presence and/or concentration of low molecular weight proteins, such as Bence Jones proteins, in a test sample. All prior art assay techniques for low molecular weight proteins involve immunoelectrophoresis methods or heat test methods that are time consuming, relatively expensive and are not amenable for use by the patient at home to detect low molecular weight proteins in urine.
Bence Jones proteins belong to a class of urinary proteins having a low molecular weight of approximately 20,000 and that are small enough to pass through the glomerular filters of the kidney. However, the Bence Jones proteins usually are reabsorbed in the tubular section of the kidney. Therefore, the concentration of Bence Jones proteins is negligible in the urine of a healthy person. As a result, a significant amount of Bence Jones proteins in urine generally is clinically significant. Overall, the detection and measurement of the concentration of low molecular weight proteins in urine is important because certain diseases are characterized by the excretion of specific low molecular weight proteins (globulins) rather than by diffuse proteinuria characterized by elevated albumin levels.
For example, the Bence Jones proteins represent a portion of the high molecular weight plasma myeloma globulin, and therefore are found in increased amounts in the urine of more than one-half of the patients suffering from multiple myeloma or leukemia. Bence Jones proteinuria also is found in the urine of many patients suffering from macroglobulinemia and primary systemic amyloidosis. In addition, an increased excretion of a specific globulin that is similar to Bence Jones proteins occurs in Franklin's disease; and patients with renal tubular disorders, such as the Fanconi syndrome, show a substantial increase in the quantity of globulins excreted in the urine. Accordingly, investigators have searched for a simple assay for low molecular weight proteins because the dye-binding method used in commercially available test strips is insensitive to low molecular weight proteins, like Bence Jones proteins. Surprisingly and unexpectedly, the method of the present invention provides a technique to detect and measure the concentration of low molecular weight proteins, like Bence Jones proteins using a dual indicator reagent composition incorporated into a polymerized urethane-containing film, layer or membrane having an appropriate pore size.
The Bence Jones proteins differ from all other urinary proteins in that they coagulate upon heating to temperatures between about 45.degree. C. and about 60.degree. C., and then redissolve on further heating to the boiling point of test sample. This peculiar characteristic of Bence Jones proteins has been the basis of all qualitative and semiquantitative determinations for Bence Jones proteins. The dye binding technique used in commercially available test strips has proved insensitive to Bence Jones proteins because the much greater relative concentration of higher molecular weight proteins, such as albumin, in the urine of a healthy individual effectively interferes with and masks the presence of Bence Jones proteins. Furthermore, it is inconvenient and costly to separate the albumin from Bence Jones proteins, thereby negating the utility of separating the albumin from the Bence Jones proteins before using a dry phase test strip.
As a result, dry phase test strips are presently unavailable to test for the presence and concentration of Bence Jones proteins in urine. However, incorporating the highly sensitive dual indicator reagent composition of the present invention into a carrier matrix having a sufficiently small pore size prevents the albumin content of the urine sample from penetrating the carrier matrix and interacting with the dual indicator reagent composition to cause a color transition. However, the carrier matrix is of sufficient pore size to allow Bence Jones proteins to penetrate the carrier matrix and to interact with the dual indicator reagent composition to cause a color transition.
Proteinuria resulting either from abnormally high albumin levels or the presence of low-molecular weight proteins depends upon the precise nature of the clinical and pathological disorder and upon the severity of the specific disease. Proteinuria can be intermittent or continuous, with transient, intermittent proteinuria usually being caused by physiologic or functional conditions rather than by renal disorders. Therefore, accurate and thorough assays of urine and other test samples for protein must be available for both laboratory and home use. The assays must permit the detection and measurement of the proteins of interest, either albumin and/or Bence Jones proteins, such that a correct diagnosis can be made and correct medical treatment implemented, monitored and maintained. In addition, it would be advantageous if the protein assay method, both for high molecular weight proteins, like albumin, and low molecular weight proteins, like Bence Jones proteins, could be utilized in a dip-and-read format for the easy and economical, qualitative and/or semiquantitative determination of protein in urine or other test samples.
Furthermore, any method of assaying for protein in urine or other test samples must yield accurate, trustworthy and reproducible results by utilizing a composition that undergoes a color transition as a result of an interaction with protein, and not as a result of a competing chemical or physical interaction, such as a pH change or preferential interaction with a test sample component other than protein. Moreover, it would be advantageous if the protein assay method is suitable for use both in wet assays and in dry reagent strips for the rapid, economical and accurate determination of protein in urine or other test samples. Additionally, the method and composition utilized in the assay for protein should not adversely affect or interfere with the other test reagent pads that are present on multiple test pad strips.
Prior to the present invention, no known method of assaying urine or other test samples for proteins included a dual indicator reagent composition that improves color resolution of the assay and increases the sensitivity of the assay at lower protein concentration levels, such that accurate and trustworthy protein assays can be made for protein concentrations of about 30 mg/dL and below. In addition, although a dry phase chemistry test strip utilizing a single dye, such as tetrabromophenol blue or tetrachlorophenol-3,4,5,6-tetrabromosulfonephthalein, has been used extensively for several years, no dry phase test strip has incorporated two dyes to improve visual color resolution, and therefore increase sensitivity, at lower protein concentration levels. Furthermore, until the method of the present invention, dry phase test strip procedures were available principally to test for total protein concentration, i.e., for albumin. However, surprisingly and unexpectedly, the method of the present invention permits the dry phase test strip assay of urine and other test samples for low molecular weight proteins, such as Bence Jones proteins.
The prior art contains numerous references on the wet phase and the dry phase chemistry utilized in the pH indicator dye method of assaying urine for proteins. For example, Keston U.S. Pat. No. 3,485,587 discloses the basic dye binding technique used to assay for proteins at a constant pH. Keston teaches utilizing a single indicator dye, maintained at a constant pH slightly below the pK.sub.a (acid dissociation constant) of the dye, to determine the presence and/or concentration of albumin by monitoring the color transition of the dye.
In contrast to the prior art, and in contrast to the presently available commercial test strips, the method of the present invention provides increased sensitivity in the detection and measurement of proteins in urine by utilizing a combination of indicator dyes, such that accurate protein levels of about 30 mg/dL and below can be determined. Unexpectedly and surprisingly, the method of the present invention also allows the simple and essentially immediate detection and measurement of low levels of Bence Jones proteins; a method heretofore impossible because of interference by the relatively high concentration of albumin in the urine sample. Hence, in accordance with the method of the present invention, new and unexpected results are achieved in the dry phase reagent strip assay, and the wet assay, of urine and other test samples for proteins, including low molecular weight proteins, by utilizing a dual indicator reagent composition incorporated into a carrier matrix having an appropriate pore size.